It is widely known that the electric resistance of a sintered zinc oxide would considerably vary depending on electric voltage, Such a material has been widely applied to the stabilization of electric voltage or to the absorption of surge voltage by taking advantage of the nonlinear relationship between its voltage and current. These electric nonlinear elements are called varistors. The quantative relationship between the electric current and voltage of a varistor is approximately represented by the following equation (1). EQU I=(V/C).sup..alpha. ( 1)
wherein V represents an electric voltage applied to the varistor; I represents an electric current passing therethrough; C is a constant; and .alpha. is an index larger than 1.
In this case, .alpha. is called a nonlinear index which indicates the degree of the nonlinearity. Generally speaking, the larger u value is the more preferable. .alpha. is calculated according to the following equation (2). EQU .alpha.=log(I.sub.2 /I.sub.1)/log(V.sub.2 /V.sub.1) (2)
wherein V.sub.1 and V.sub.2 each represent the electric voltage at given current I.sub.1 and I.sub.2.
In a common case, I.sub.1 and I.sub.2 are determined a 1 mA and 10 mA, respectively, and V.sub.1 is called the varistor voltage. C and .alpha. vary depending on the formulation and production method of the varistor. These facts have already been well known in the art.
A zinc oxide varistor may be usually produced by the following method.
Namely, additives are mixed with zinc oxide and dried. The obtained dried matter is molded into a desired shape by a common molding method employed for ceramics and subsquently sintered at an appropriate temperature. During this sintering stage, required reactions would occur among the zinc oxide and additives. Thus the mixture is molten and sintered to thereby give the aimed varistor material. Subsequently the obtained varistor material is provided with electrodes and a conductor. Thus an element is formed.
Although several theories have been reported relating to the mechanisms of the expression of the varistor properties of sintered zinc oxide materials, no definite one has been established so far. However it is recognized that the electric properties of a varistor originate from its microstructure. A zinc oxide varistor generally comprises zinc oxide particles around which a highly resistant boundary layer is located and bound thereto. Additives are employed in order to form this boundary layer. Several or more additives are generally used and the types and amounts thereof may vary depending on the aimed properties.
Conventional methods for the production of a zinc oxide varistor material suffer from a serious problem. That is to say, the properties of a sintered material would widely vary, which makes it impossible to efficiently produce varistor materials of constant properties. This problem might be caused by the fact that there are a number of additives to be used and these additives complicatedly and delicately react with zinc oxide as well as with each other upon firing. Therefore, these reactions are considerably affected by a change in the production conditions. Thus, it is highly difficult to uniformly control the microstructure of the sintered material and the microdistribution of chemical components thereof at a high reproducibility.
Furthermore, additives which are liable to be evaporated at a high temperature such as bismuth oxide are frequently employed in the prior art, which makes the control of the microstructure of the sintered material and microdistribution of chemical components thereof more difficult.